Attaching/detaching question

Attaching/detaching question

I merged a file into my tree using FTM for mac. I noticed that some individuals no longer listed parents or children, although that information was in my original tree. In other words, Tom W. is the son of Tom W Sr.but when I display Tom W, it doesnt show his father. I tried to attach but the program tells me that it can't as there is already a relationship between the two.... even though the dad isn't showing up . WHat did I do wrong? and how to fix please.